The present invention relates to a homogenous charge compression ignition engine (HCCI engine). In the HCCI engine of the present invention, the combustion mode can be switched between homogenous charge compression ignition combustion (HCCI combustion) and spark ignition combustion (SI combustion).
In recent years, homogenous charge compression ignition (HCCI) engines have been drawing attention, and various researches have been conducted on such engines. Excellent fuel economy and thermal efficiency and low emission can be gained from the HCCI engines. In some kinds of HCCI engines, fuel is directly injected into a combustion chamber during an intake stroke. That is, only air is drawn into the combustion chamber from an intake passage and fuel is mixed with air for the first time in the combustion chamber. However, in most of the HCCI engines, the fuel is mixed with air on the intake passage so as to generate air-fuel mixture. The air-fuel mixture is supplied to the combustion chamber from the intake passage.
The temperature increases and the pressure rises in the air-fuel mixture contained inside the combustion chamber as a piston rises at the time of the compression stroke of the engine, so that the mixture spontaneously ignites. One obstacle that needs to be overcome in terms of putting HCCI engines into practice is that the engine operating range, which allows homogenous charge compression ignition (HCCI) combustion to be stably controlled, is still narrow. In order to overcome the obstacle, there is a trend to implement HCCI combustion in stationary engines where the ordinarily used operating range is relatively narrow, for example, gas engines for GHPs (gas heat pumps).
In an actual operation of the HCCI engines, an engine low rotation speed range, a middle rotation speed range, a low load range and a middle load range are frequently used. It has been proposed that the combustion mode is switched between the HCCI combustion range and the spark ignition (SI) combustion range according to the running state of the engine. The spark ignition (SI) combustion is carried out in the high rotation speed range, the extremely low load range, and the high load range.
Japanese Laid-Open Patent Publication No. 2007-16685 discloses a controlling method in which, which switching from the spark ignition combustion to the HCCI combustion, the internal EGR amount is increased at the same time as making the air-fuel ratio leaner. When switching from the spark ignition combustion to the HCCI combustion, the intake air amount to the engine is increased when the combustion mode is still in the spark ignition combustion, thereby making the air-fuel ratio leaner. At the same time as making the air-fuel ratio leaner, the internal EGR amount is increased. Thereafter, the spark ignition combustion is switched to the HCCI combustion via the stratified combustion.
According to the controlling method of the above publication, the internal EGR gas is caused when the combustion mode is still in the spark ignition combustion, that is, when the HCCI combustion is still difficult to carry out. Since the internal EGR gas has a high temperature, the spark ignition combustion and the HCCI combustion might be simultaneously carried out. This can lead to a significant torque fluctuation.
Further, in the controlling method of the publication, the stratified charge combustion is temporarily performed when switching from the spark ignition combustion to the HCCI combustion. The stratified charge combustion is effective in increasing the throttle opening degree in a state where the air-fuel ratio has been made leaner to improve the ignitability of the spark ignition combustion. However, the stratified charge combustion requires an “in-cylinder injector”, which directly injects fuel into the combustion chamber. That is, an in-cylinder direct injection system is necessary that includes an in-cylinder injector, which injects fuel at high pressure. The above publication thus has drawbacks in terms of the manufacturing costs and power loss. Further, the transitional execution of the stratified charge combustion eliminates some of the advantages of the HCCI combustion, namely the improvement of the fuel economy and the reduction in emission.
An objective of the present invention is to suppress the occurrence of abrupt changes in the torque of the HCCI engine when the combustion mode is switched from the spark ignition combustion to the HCCI combustion.
In accordance with one aspect of the present invention, a homogenous charge compression ignition engine (HCCI) is provided. The HCCI engine is capable of switching the combustion mode between the HCCI combustion and the spark ignition combustion. The HCCI engine includes a combustion chamber and a piston that reciprocates in the combustion chamber. The piston defines an exhaust top dead center of the combustion chamber. The HCCI engine includes an intake valve and an exhaust valve. An intake variable valve mechanism changes an intake lift amount, which is a lift amount of the intake valve. The intake lift amount is set to a first intake lift amount in the spark ignition combustion and to a second intake lift amount in the HCCI combustion. The intake variable valve mechanism is capable of controlling an intake opening timing, which is the opening timing of the intake valve. An exhaust variable valve mechanism changes an exhaust lift amount, which is a lift amount of the exhaust valve. The exhaust lift amount is set to a first exhaust lift amount in the spark ignition combustion and to a second exhaust lift amount in the HCCI combustion. The exhaust variable valve mechanism is capable of controlling an exhaust closing timing, which is the closing timing of the exhaust valve. A reference exhaust closing timing to be used in the HCCI combustion is set. A controller controls the intake variable valve mechanism and the exhaust variable valve mechanism. The controller sets an internal EGR amount, which includes the amount of burned gas that remains in the combustion chamber at the time of the HCCI combustion, by setting a negative valve overlap period, during which both of the intake valve and the exhaust valve are closed, such that not all the burned gas is discharged from the combustion chamber. When switching the combustion mode from the spark ignition combustion to the HCCI combustion, the controller executes following operations a), b), and c):
a): switching the intake lift amount from the first intake lift amount to the second intake lift amount, such that the intake opening timing is delayed relative to the exhaust top dead center;
b): switching the exhaust lift amount from a first exhaust lift amount to a second exhaust lift amount after the operation a); and
c) delaying the exhaust closing timing relative to the reference exhaust closing timing, such that the internal EGR amount is generated.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.